Among the desirable properties of a photographic silver halide recording material is high sharpness. That is, the recording material should enable faithful reproduction and display of both coarse and fine details of the original scene. This combination of properties has proven difficult to achieve in practice.
A general description of the nature of this problem may be found in T. H. James, Ed., "The Theory of the Photographic Process," Macmillan, N.Y., 1977 and, in particular, at Chapter 20 of this text, pages 578-591, entitled "Optical Properties of the Photographic Emulsion" by J. Gasper and J. J. DePalma.
One method of improving sharpness, disclosed at U.S. Pat. No. 4,312,941 and at U.S. Pat. No. 4,391,884, involves the incorporation of a spatially fixed absorber dye in a film layer between the exposing light source and a layer comprising a conventional grain light sensitive silver halide emulsion. In these disclosures, the absorber dye is held spatially fixed either by means of a ballast group or by means of a mordanting material incorporated at a specified position in the film structure. Use of this spatial arrangement of absorber dye and emulsion reduces front-surface halation effects.
U.S. Pat. No. 4,439,520, inter alia, discloses the utility of sensitized high aspect ratio silver halide emulsions for use in light sensitive materials and processes. These high aspect ratio silver halide emulsions, herein known as tabular grain emulsions, differ from conventional grain emulsions in many characteristics. One differential characteristic is the relationship between the emulsion grain thickness and the emulsion grain equivalent circular diameter. Conventional grain emulsions tend to be isotropic in shape and, when incorporated in a film structure, tend to be randomly oriented within a particular layer. Tabular grain emulsions however, tend to be anisotropic in shape and, when incorporated in a film structure, tend to align such that their major axis parallels the plane of the film base. This degree of anisotropicity is know as the emulsion aspect ratio (AR), typically defined as the ratio of the emulsion grain equivalent circular diameter divided by the emulsion grain thickness. The ability to control emulsion grain thickness and alignment within a film structure can enable the realization of otherwise unattainable degrees of recording material performance.
The optical properties of photographic recording materials incorporating tabular grain emulsions are described in great detail at "Research Disclosure", No. 25330, May, 1985, as are methodologies of specifying particular arrangements of tabular grain emulsions within a film structure and of specifying particular tabular grain emulsion thicknesses so as to enable the attainment of specifically desired properties, such as speed or sharpness in underlying or overlying emulsion layers.
These methods may not prove to be wholly satisfactory. U.S. Pat. No. 4,740,454, for example, discloses that although high frequency sharpness may be attained by the appropriate choice of tabular grain emulsion thickness and placement, this can be at the cost of low frequency sharpness. The term "high frequency sharpness" generally relates to the appearance of fine detail in a scene reproduction, while the term "low frequency sharpness" generally relates to the appearance of clarity or "snap" in scene reproduction. It is understood that the terms "high frequency sharpness" and "low frequency sharpness" are qualitative in nature and that both image spatial frequency, expressed as cycles/mm in the film plane, and the image magnification employed in producing a reproduction must be taken into account when specifying such terms. This publication discloses that both high frequency and low frequency sharpness may be simultaneously improved by the incorporation of specific mercaptothiadiazole compounds in combination with tabular grain silver halide emulsions. This practice may not be wholly satisfactory since the incorporation of such silver ion ligands can lead to deleterious effects on film speed and film keeping properties.
In a related area, U.S. Pat. Nos. 4,746,600 and 4,855,220 disclose that unexpectedly large degrees of sharpness can be attained by combining spatially fixed absorber dyes and Development Inhibitor Releasing Compounds (DIR Compounds) in a photographic silver halide recording material. The spatially fixed absorber dye is positioned between an emulsion containing layer and the exposing light source. The materials described in these disclosures incorporate either conventional grain silver halide emulsions or low aspect ratio tabular grain silver halide emulsions. There is no indication of any dependence in film imaging performance on the thickness or spatial positioning of the light sensitive silver halide emulsion grains in these publications.
Again, in a related area, U.S. Pat. No. 4,833,069 discloses that large degrees of sharpness can be attained by simultaneously controlling imaging layer thickness to between 5 and 18 microns and incorporating large quantities, between 15 and 80 mol % of colored cyan dye-forming couplers, known also in the art as cyan dye-forming color masking couplers. This method may not be wholly satisfactory since the use of excessive quantities of color masking couplers can lead to inferior color rendition by over-correcting the color reproduction through excessive use of the masking function. Again, there is no indication of any dependence in film imaging performance on the thickness or spatial positioning of the light sensitive silver halide emulsion grains as described in this publication.
In yet another related area, U.S. Pat. No. 4,956,269 discloses that color reversal silver halide photographic materials incorporating tabular grain silver halide emulsions can show improved sharpness when the photographic layer incorporating the tabular grain silver halide emulsion also incorporates a quantity of absorber dye sufficient to reduce the speed of that layer by at least 20%, when the total imaging layer thickness is less than 16 microns and when the swell ratio of the film is greater than 1.25. The materials described in this disclosure incorporate intermediate aspect ratio (AR&lt;9.0) tabular grain silver halide emulsions. The criticality of color dye forming Development Inhibitor Releasing (DIR) coupler presence and quantity is not addressed in this publication. As is generally known in the art, color reversal films are designed to operate in a best mode when developed according to a color reversal process while color negative films are designed to operate in a best mode when developed according to a color negative process. Color reversal processing entails a non-chromogenic "first development" step utilizing a potent silver halide solvent and a total grain developer, followed ultimately by a "second development" step producing total grain color development that employs a paraphenylene diamine developer, facilitated by unusually high developer solution pH. In profound contrast, a typical color negative film development step comprises a low silver halide solvent, grain surface developer that proceeds only to partial completion (typically 15-20% conversion).
It is the purpose of this color reversal material total development process to produce a unique density response as a function of the logarithm of exposure, known as a characteristic curve, with very high maximum density and very low minimum density over a relatively short imagewise exposure range so as to produce a high contrast positive image suitable for direct viewing. As a consequence of both the high range of density extreme and the smaller imaging range or latitude of the color reversal material, there is virtually no regime in the characteristic curve of constant contrast (or gamma) or straight-line curve shape that would linearly record scene luminance. Color negative materials differ graphically by producing long, imagewise straight-line characteristic curve segments at much lower contrast.
It is well appreciated by those skilled in the art that in order to achieve their unique characteristic curve responses, color reversal photographic recording materials comprise strikingly different varieties and quantities of photographic imaging constituents than color negative recording materials. In particular, materials intended for reversal processing typically contain much less silver halide and gelatin than color negative films. As a consequence largely of lower silver halide content, reversal color recording materials are thinner than color negative recording materials, as noted in Mitchell's "Photographic Science", John Wiley and Sons, 1984, at pages 194-195. Other substantial differences, totally neglecting the matters of silver halide grain composition, morphology, and spectrochemical sensitizations for vastly different development conditions, relate to the methods of color correction for faithful scene color reproduction. Materials intended for reversal processing and direct viewing do not comprise the colored masking couplers that are ubiquitous in the color negative recording materials of the commercial art due to their deleterious effect on the lightness and hue of scene highlights that the corrected reversal material would render.
Further, as discussed in Neblette's "Imaging Processes and Materials", 8th edition, 1988 at page 127, color reversal films generally include only colorless DIRs, which release development inhibitors as a result of a cross oxidation reaction with oxidized hydroquinone species present in the non-coupling first developer step employed in a color reversal process. This non-coupling step is generally absent from a color negative image forming process. Color dye forming DIR couplers are designed to not liberate development inhibitor in a non-coupling development step. For these reasons, the conditions and constraints relevant to a color reversal material designed to operate in a color reversal process are non-predictive of the performance of color negative silver halide photographic materials.
A color negative silver halide photographic recording material incorporating conventional grain silver halide emulsions and a quantity of distributed dye sufficient to reduce the speed of a color record by about 50% has been commercially available for many years. Additionally, it has been common practice in the photographic art to commercially provide silver halide photographic recording materials incorporating conventional grain and/or tabular grain silver halide emulsions in combination with soluble dyes sufficient to reduce the speed of a color record by about 10% for purposes related to ease of manufacture. Likewise, color negative silver halide photographic materials incorporating high aspect ratio tabular grain silver halide emulsion with an average grain thickness of circa 0.11 and 0.14 microns in an intermediately positioned layer have been commercially available for many years.